Mixer circuit

ABSTRACT

A frequency components signal LO of a local oscillation wave is input to the base of a transistor ( 1 ) configuring a local frequency multiplication unit ( 10 ) and a signal (↓LO) opposite in phase to the local oscillation wave is input to the base of a transistor ( 2 ). The transistors ( 1, 2 ) have their collectors connected together and their emitters connected together, and each collector outputs a modulation signal. A reference transistor ( 3 ) is differentially connected to the transistors ( 1, 2 ). A load ( 6 ) is connected to the collector of each transistor ( 1 - 3 ) serving as an output of differential connection. Each differentially connected transistor ( 1 - 3 ) has its emitter with a collector connected thereto. A transistor ( 4 ) has its base receiving a modulated wave signal, and its emitter connected to a constant current source ( 5 ). A mixer circuit is thus configured.

TECHNICAL FIELD

[0001] The present invention relates generally to mixer circuits andparticularly to a direct conversion mixer for use in mobilecommunications.

BACKGROUND ART

[0002] A direct conversion system is known which converts a signalmodulated into a radio frequency directly to a baseband signal andmodulates a baseband directly to a radio frequency. Generally, thissystem does not provide an intermediate frequency. As such, it candispense with image suppression, reduce spurious source, an d simplify asystem advantageously. It is also known, however, that the systemprovides 2nd order intermodulation distortion, local oscillation wavenoise, and other similar disadvantages. One approach to alleviate thesedisadvantages is an even harmonics mixer such as an anti-parallel diode.

[0003] In recent years there is a demand for miniature and inexpensiveICs for high-frequency radio communications, as represented for exampleby mobile phones. In such a market, current consumption reduction, ICyields and the like are important issues.

[0004] In an even harmonics mixer, a mixer circuit itself does notconsume a current. To operate the mixer circuit, however, a high localoscillation wave input level is required. Accordingly, overall currentconsumption reduction has been an issue to be addressed. Furthermorethere is also a demand for reducing an influence of semiconductorprocess variation.

[0005] As an approach to overcome such disadvantages, a system using atransistor's diode characteristics has been considered in recent years.

[0006]FIG. 5 is a circuit diagram showing one example of a conventionaleven harmonics mixer using transistors. As shown in the figure,transistors 1 and 2 have their respective collectors connected togetherand their respective emitters connected together to configure a localfrequency multiplication unit. A resistor R1 is connected between thecollectors and a power supply and a resistor R2 is connected between theemitters and ground. Transistors 1 and 2 receive a modulated wave signalBB at their respective emitters and receive signals LO and ↓LO (“↓”indicates an inverted signal) each having a component of a frequency ofa local oscillation wave inverted in phase at their respective bases.Thus the local frequency multiplication unit generates a localoscillation wave frequency component multiplied by two and can extract adesired frequency component by multiplying an input signal frequency andthe local oscillation wave frequency component.

[0007] The mixer of FIG. 5, however, is associated with a small powergain and requires a high local oscillation wave input to operate thelocal frequency multiplication unit.

DISCLOSURE OF THE INVENTION

[0008] A main object of the present invention is therefore to provide inthe field of high-frequency radio communications a mixer circuitsuitable for applications for which there is a demand for low frequencycurrents, high IC yields and simplified system configurations.

[0009] In accordance with the present invention a mixer circuitincludes: a local frequency multiplication unit configured of a firsttransistor having an input electrode receiving a frequency component ofa local oscillation wave signal and a second transistor receiving asignal opposite in phase to the local oscillation wave, the first andsecond transistors having their respective first electrodes connectedtogether and their respective second electrodes connected together, thelocal frequency multiplication unit outputting a modulation signal atthe first electrode of each transistor; a third transistor connected ina pair with each of the first and second transistors of the localfrequency multiplication unit, having an input electrode receiving areference signal, and a first electrode differentially outputting themodulation signal; a voltage source providing the first, second andthird transistors at their respective first electrodes with apredetermined voltage; a signal input unit connected to the first,second and third transistors at their respective second electrodes andreceiving a modulated signal; and a constant current source connected tothe signal input unit.

[0010] Furthermore, the signal input unit is a fourth transistor havingan input electrode receiving the modulated signal, a first electrodeconnected to the second electrode of each of the first, second and thirdtransistors, and a second electrode connected to the constant currentsource.

[0011] Furthermore, there are provided two sets of the local frequencymultiplication unit, the third transistor for reference and the fourthtransistor, the voltage source is connected to the two local frequencymultiplication units commonly, and the constant current source suppliesa constant current to the two fourth transistors commonly.

[0012] Furthermore, the constant current source is a variable constantcurrent source.

[0013] Furthermore, the mixer circuit further includes a variablecurrent source varying a bias current provided to the input electrode ofthe third transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a circuit diagram of a mixer circuit of one embodimentof the present invention;

[0015]FIG. 2 is a circuit diagram of another embodiment of the presentinvention;

[0016]FIG. 3 is circuit diagram showing another embodiment of thepresent invention;

[0017]FIG. 4 is a circuit diagram showing still another embodiment ofthe present invention; and

[0018]FIG. 5 is a circuit diagram showing one example of an evenharmonics mixer using a conventional transistor.

BEST MODES FOR CARRYING OUT THE INVENTION

[0019]FIG. 1 is a circuit diagram of a mixer circuit of one embodimentof the present invention. In FIG. 1, similarly as shown in theconventional example of FIG. 5, transistors 1 and 2 have theirrespective collectors connected together and their respective emittersconnected together, each collector receiving a power supply via aresister R3, and their respective bases receiving frequency componentssignals LO and ↓LO of a local oscillation wave to configure a localfrequency multiplication unit 10. Local frequency multiplication unit 10generates in a manner similar to that of an even harmonics mixer afrequency corresponding to the local oscillation wave frequencymultiplied by two.

[0020] Furthermore, a reference transistor 3 has its emitter connectedto the emitters of transistors 1 and 2, its collector connected to apower supply via a resistor R4, and its base receiving a bias currentRef.

[0021] Resistors R3 and R4 configure a load 6 serving as a voltagesource. In place of resistors R3, R4 a different type of load may beconnected. Transistors 1, 2 and 3 have their emitters connected to thecollector of a transistor 4 for receiving a modulated wave. Transistor 4has its emitter receiving a constant current from a constant currentsource 5 and its base receiving a modulated wave signal BB.

[0022] In the mixer circuit of FIG. 1, the sum of the current flowingthrough local frequency multiplication unit 10 configured of transistors1 and 2 and that flowing through transistor 3 equals that flowingthrough transistor 4. Furthermore, the current flowing throughtransistor 4 equals the constant current of constant current source 5.As such by allowing transistor 4 to have a gain a mixer circuit having again can be implemented.

[0023] The frequency multiplied by two and generated by local frequencymultiplication unit 10 configured of transistors 1 and 2 varies thecurrent of local frequency multiplication unit 10. Since transistor 4provides a constant-current operation relative to local frequencymultiplication unit 10 and transistor 3, the variation in currentresults in transistor 3 having a current of opposite phase flowingtherethrough. Transistor 4 has a gain for a modulated wave that isreceived and the amplified signal is converted in frequency by localfrequency multiplication unit 10. The signal converted in frequency isoutput at load 6 as differential output signals RF and ↓RF.

[0024] Thus in the present embodiment a mixer circuit configured oflocal frequency multiplication unit 10 configured of transistors 1 and 2and an input unit formed of transistor 4 to receive a modulated wave isprovided with reference transistor 3 paired with local frequencymultiplication unit 10 to provide a differential mixer. Withoutincreasing an input level of a local oscillation wave a large-gain mixercircuit can be implemented.

[0025]FIG. 2 is a circuit diagram of another embodiment of presentinvention. In FIG. 2, local frequency multiplication unit 10 andtransistors 3 and 4 are similar in configuration to those shown inFIG. 1. A local frequency multiplication unit 20 and transistors 9, 11are configured similarly as shown in the mixer circuit of FIG. 1. Morespecifically, transistors 7 and 8 have their respective collectorsconnected together, their respective emitters connected together, andtheir respective bases receiving frequency component signals LO, ↓LO ofa local oscillation wave, respectively.

[0026] Transistors 7, 8 have their respective emitters connected to theemitter of a reference transistor 9. Reference transistor 9 has its basereceiving a bias current. Each transistor 7, 8, 9 has its emitterconnected to the collector of a transistor 11 receiving a modulatedwave. Transistor 11 has its base receiving a modulated wave signal ↓BBopposite in phase to the modulated wave signal BB input to the base oftransistor 4.

[0027] Local frequency multiplication unit 20 has its collectorsconnected to collectors of local frequency multiplication unit 10, andeach collector receives a power supply commonly via a resistor R3.Transistor 9 has its collector connected to a collector of transistor 3and these collectors receive a power supply via a resistor R4.Transistor 11 has its emitter connected to an emitter of transistor 4and these emitters are connected to constant current source 5 commonly.In other words, the mixer circuit of FIG. 2 corresponds to the mixercircuit of FIG. 1 if the mixer circuit of FIG. 1 has the Gilbert cellconfiguration.

[0028] Thus in the present embodiment each mixer circuit having the sameconfiguration as shown in FIG. 1 can have a large gain, asaforementioned, and it can also realize the Gilbert cell configurationto implement a mixer circuit capable of suppressing a frequencycomponent of a local oscillation wave leaking toward an output.

[0029]FIG. 3 is a circuit diagram showing another embodiment of thepresent invention. In the present embodiment, constant current source 5shown in FIG. 1 is replaced by a variable current source 12. A variablegain control effect can be obtained.

[0030]FIG. 4 is a circuit diagram showing still another embodiment ofthe present invention. In the present embodiment, a variable currentsource 13 is connected to the base of the reference transistor 3 of FIG.1 to allow a bias current to be variable. A variable gain control effectcan be obtained.

[0031] The above embodiments have described a mixer circuit mixing afrequency component of a local oscillation wave and the signal of amodulated wave BB together to output a signal of a modulated wave. Themixer circuit may receive a radio frequency signal having been receivedas a signal of a modulated wave and mix it with a frequency component ofa local oscillation wave, and output a baseband signal as a modulationsignal.

[0032] Furthermore while the above embodiments are configured of abipolar transistor, they may be configured of a field effect transistor.

[0033] Thus in the present invention a mixer circuit configured of alocal frequency multiplication unit and an input unit receiving amodulated wave is provided with a reference transistor to provide adifferential mixer thereby to simplify a system configuration with largegain. Furthermore, constant-current control can reduce dependency onsemiconductor process variation and providing a constant current sourcein a bandgap configuration can reduce power supply voltage variation andtemperature variation.

[0034] Industrial Applicability

[0035] In accordance with the present invention a mixer circuit can haveboth of an advantage of a conventional mixer circuit and that of an evenharmonics mixer to allow the even harmonics mixer to be active toimplement a large-gain mixer circuit while preventing an influence of acarrier component. It is applicable to mobile communications involvingmodulation from a baseband signal directly to a radio frequency.

1. A mixer circuit comprising: a local frequency multiplication unit(10) configured of a first transistor (1) having an input electrodereceiving a frequency component of a local oscillation wave signal and asecond transistor (2) receiving a signal opposite in phase to said localoscillation wave, said first and second transistors having theirrespective first electrodes connected together and their respectivesecond electrodes connected together, said local frequencymultiplication unit (10) outputting a modulation signal at said firstelectrode of each said transistor; a third transistor (3) connected in apair with each of said first and second transistors of said localfrequency multiplication unit, having an input electrode receiving areference signal, and a first electrode differentially outputting saidmodulation signal; a voltage source (6) providing said first, second andthird transistors at their respective first electrodes with apredetermined voltage; a signal input unit connected to said first,second and third transistors at their respective second electrodes andreceiving a modulated signal; and a constant current source (5)connected to said signal input unit.
 2. The mixer circuit according toclaim 1, wherein said signal input unit is a fourth transistor (4)having an input electrode receiving said modulated signal, a firstelectrode connected to said second electrode of each of said first,second and third transistors, and a second electrode connected to saidconstant current source.
 3. The mixer circuit according to claim 1,wherein: there are provided two sets of said local frequencymultiplication unit (10, 20), said third transistor for reference (3, 9)and said fourth transistor (4, 11); said voltage source is connected tosaid two local frequency multiplication units commonly; and saidconstant current source (5) supplies a constant current to said twofourth transistors commonly.
 4. The mixer circuit according to claim 2,wherein said constant current source is a variable constant currentsource (12).
 5. The mixer circuit according to claim 1, furthercomprising a variable current source (13) varying a bias currentprovided to said input electrode of said third transistor.